In optics, a diffraction grating is an array of fine, parallel, equally spaced grooves (“rulings”) on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called “orders,” or “spectral orders.”
The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter.
Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle θ, the deviation of the diffracted ray from the direction predicted by geometric optics, is given by the following equation, where m is the spectral order, λ is the wavelength, and d is the spacing between corresponding parts of adjacent grooves:
  θ  =      ±                  sin                  -          1                    ⁡              (                              m            ⁢                                                  ⁢            λ                    d                )            
Because the angle of deviation of the diffracted beam is wavelength-dependent, a diffraction grating is dispersive, i.e. the diffraction grating separates the incident beam spatially into its constituent wavelength components, producing a spectrum.
The spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order. Diffraction gratings are often used in monochromators and other optical instruments. By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called “blazing.”
Originally high resolution diffraction gratings were ruled. The construction of high quality ruling engines was a large undertaking. A later photolithographic technique allows gratings to be created from a holographic interference pattern. Holographic gratings have sinusoidal grooves and so are not as bright, but are preferred in monochromators because they lead to a much lower stray light level than blazed gratings. A copying technique allows high quality replicas to be made from master gratings, this helps to lower costs of gratings.
A planar waveguide reflective diffraction grating includes an array of facets arranged in a regular sequence. The performance of a simple diffraction grating is illustrated with reference to FIG. 1. An optical beam 1, with a plurality of wavelength channels λ1, λ2, λ3 . . . , enters a diffraction grating 2, with grading pitch Λ and diffraction order m, at a particular angle of incidence θin. The optical beam is then angularly dispersed at an angle θout depending upon wavelength and the order, in accordance with the grating equation:mλ=Λ(sin θin+sin θout)  (1)
From the grating equation (1), the condition for the formation of a diffracted order depends on the wavelength λN of the incident light. When considering the formation of a spectrum, it is necessary to know how the angle of diffraction θNout varies with the incident wavelength θin. Accordingly, by differentiating the equation (1) with respect to θNout, assuming that the angle of incidence θin is fixed, the following equation is derived:∂θNout/∂λ=m/Λ cos θNout  (2)
The quantity dθNout/dλ is the change of the diffraction angle θNout corresponding to a small change of wavelength λ, which is known as the angular dispersion of the diffraction grating. The angular dispersion increases as the order m increases, as the grading pitch Λ decreases, and as the diffraction angle θNout increases. The linear dispersion of a diffraction grating is the product of this term and the effective focal length of the system.
Since light of different wavelengths λN are diffracted at different angles θNout, each order m is drawn out into a spectrum. The number of orders that can be produced by a given diffraction grating is limited by the grating pitch Λ, because θNout cannot exceed 90°. The highest order is given by Λ/λN. Consequently, a coarse grating (with large Λ) produces many orders while a fine grating may produce only one or two.
A blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a blaze angle w, as shown in FIG. 1. The selection of the blaze angle w offers an opportunity to optimize the overall efficiency profile of the diffraction grating, particularly for a given wavelength.
Planar waveguide diffraction based devices provide excellent performance in the near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM). In particular, advancements in Echelle gratings, which usually operate at high diffraction orders (40 to 80), high angles of incidence (approx 60°) and large grading pitches, have lead to large phase differences between interfering paths. Because the size of grating facets scales with the diffraction order, it has long been considered that such large phase differences are a necessity for the reliable manufacturing of diffraction-based planar waveguide devices. Thus, existing devices are limited to operation over small wavelength ranges due to the high diffraction orders required.
Reflective diffraction gratings, etched directly into a planar lightwave circuit, are often used as wavelength filters due to their high performance and small size. Conventional PLCs can be fabricated on a number of different types of substrates, including silica-on-silicon, silicon-on-insulator (SOI), or indium phosphide (InP). A typical configuration of a diffraction grating filter formed at a side of a slab waveguide is shown in FIG. 1. It is assumed that all the action is in a two-dimensional plain parallel to the plane of the page, i.e. the light is confined in the vertical direction (perpendicular to the page).
Another system is, illustrated in FIGS. 2 and 3, in which a concave reflective diffraction grating 10 is formed at an edge of a slab waveguide 11 provided in chip 12. An input port is defined by an end of a waveguide 13, which extends from an edge of the chip 12 to the slab waveguide 11 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels (λ1, λ2, λ3 . . . ), thereto. The light enters through the input port into the two-dimensional slab waveguide 11, and expands horizontally, i.e. diverges in the horizontal plane. Subsequently, the light encounters the reflective grating 10, which is composed of a number of small reflective facets. The first-order reflected signals combine constructively at one location, based on the wavelength of light, where an end of an output waveguide 15 is positioned to capture the wavelength channel of interest.
The diffraction grating 10, as defined in U.S. Pat. No. 7,151,635 issued Dec. 19, 2006 to Enablence Technologies Inc, which is incorporated herein by reference, and as illustrated in FIG. 2, has an aspect ratio (F/S) greater than 3, preferably greater than 5 and potentially greater than 10, and a sidewall length S less than or equal to the average wavelength of the wavelength channels (λ1, λ2, λ3 . . . ). The input waveguide 13 is positioned to ensure that the incident angle θin is less than 45°, preferably less than 30° and potentially less than 15° or even less than 6°, and the grating pitch Λ is selected to ensure that the grating 10 provides diffraction in an order of 5 or less and preferably 3 or less. The diffraction grating 10 disperses the input signal into constituent wavelengths and focuses each wavelength channel on a separate output port in the form of the ends of the output waveguide 15, which are disposed along a focal line 16 of the grating 10 defined by a Rowland circle, for transmission back to the edge of the chip 12. The illustrated device could also be used to multiplex several wavelength channels, input the waveguides 15, into a single output signal transmitted out to the edge of the chip 12 via the input waveguide 13. The input and output ports represent positions on the slab waveguide 11 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off.
One of the greatest challenges in fabricating a reflective diffraction grating, such as that shown in FIGS. 1 and 2, in a PLC, is the very high quality etching required to produce the small reflective facets. There are two main challenges which must be overcome to fabricate an efficient grating, i.e. a near perfect verticality of etch, and a very smooth sidewall. The grating teeth shown in FIG. 2 would typically be metallized to improve their reflectivity. However, since the light travels in the underlying silica, it is reflected off the inner metal, which conforms around all the roughness and non-verticality of the silica etch, resulting in performance problems for the grating. The only way to eliminate this problem is to develop a very high-quality vertical etch, with very low roughness.
Unfortunately, in most etch processes there is typically a tradeoff in terms of etch verticality versus roughness of the etched wall, contrary to what is necessary for making a good grating. This is true in most material systems; however, recent developments in Deep Reactive Ion Etching (DRIE) of Silicon have allowed for extremely deep, vertical, smooth etches, when implemented in silicon only. The DRIE process has become very common for use in MEMs components and many other applications.
However, using silicon as a PLC waveguide is very restrictive, and typically results in a low-performance component. To achieve the high-performance, low-loss components required in modern telecommunication systems, most PLC filter chips are fabricated in silica-on-silicon substrates, where the light travels only in a thin glass layer on top of the silicon. DRIE technology can be applied to silica wafers, but the etch results are not nearly as good as those found in silicon. For that reason, virtually all reflective diffraction gratings etched in silica suffer from performance problems associated with the verticality and/or roughness of the etched mirrors.
An object of the present invention is to overcome the shortcomings of the prior art by providing a hybrid PLC device in which a highly precise diffraction grating is manufactured separately from a high quality waveguide structure.